This invention relates generally to fiber optic acoustic sensors and particularly to fiber optic acoustic sensors for underwater use. Still more particularly, this invention relates to apparatus and methods for providing a polarization-insensitive fiber optic interferometric sensor suitable for use as a hydrophone.
A single mode optical fiber is capable of guiding signals of two linear polarizations. In a fiber optic hydrophone comprised of fiber optic interferometric sensors, the output signal may be a function of the polarization of the optical signals guided by the optical fibers in the sensors. Generally the maximum signal is obtained if the two fibers guide signals of the same polarization.
A major problem of fiber optic interferometric sensors is loss of signal due to signal fading caused by changes in the polarization of the waves guided by the fibers. Drift in the relative optical path length difference in the interferometer arms causes the path length to go into and out of quadrature, which changes the signal intensity. Complete polarization fading occurs when the polarizations are orthogonal. In prior art systems, polarization signal fading may degrade the signal intensity to the point that no usable information is available.
Strumming noise occurs when the polarization input to the hydrophone interferometer is modulated. Such modulation may be caused by vibration of the input lead or by the cable used to tow a hydrophone array through water. Strumming noise changes the stress distribution in the optical fibers that comprise the interferometer, and the changes in stress distribution change the polarization of waves guided by the fibers.
Polarization fading and strumming noise have been observed regularly in sea trials of fiber optic towed hydrophone arrays. Polarization fading in such hydrophone arrays occurs when the two fibers that comprise the hydrophone have orthogonal polarization components. Polarization fading may reduce the fringe visibility in the output of interferometric sensors to zero. All hydrophone signal information then disappears.
A fiber optic polarization controller has been used to overcome the problem of polarization signal fading. This polarization controller has a plurality of loops of the optical fiber wound on spools whose edges are mounted on a common axis. The axis lies in the plane of each coil. Adjusting the angles of the loops of optical fiber adjusts the polarization state of the optical signal guided by the fiber.
Birefringence can be induced in a single mode optical fiber by bending the fiber into a coil. Bending an optical fiber causes an increase in the material density in the plane perpendicular to the plane of the coil, which increases the refractive index in that plane. Changes in the refractive index in the plane of the coil are negligible due to the opposite effect of compression on the interior and tension on the exterior part of the curvature.
The prior art has the disadvantage of requiring a manual adjustment every few minutes. This is not practical for a multisensor array that may contain seven or more sensors because such an array would be cumbersome and bulky. A fiber optic polarization controller has been used to overcome the problem of polarization signal fading. This polarization controller has a plurality of loops of the optical fiber wound on spools whose edges are mounted on a common axis. The axis lies in the plane of each coil. Adjusting the angles of the loops of optical fiber adjusts the polarization state of the optical signal guided by the fiber.